Abstract

We present an experimental technique that allows the simultaneous spatial imaging and spectral analysis of falling droplets that exhibit lasing. Single droplet investigations serve as, among other purposes, a preliminary study for spray and combustion researchers. The described setup provides a valuable tool for the evaluation of microdroplet investigations with laser-spectroscopic techniques that rely on laser-induced fluorescence (LIF) or similar spectroscopical phenomena. The emphasis is that both spatial and spectral information are obtained from single-shot images of a falling droplet. Furthermore, combining spatial imaging and a spatially resolving optical multichannel analyzer makes a pointwise rastering of the droplets spectrum possible. This allows for the (almost) unambiguous determination of sources of influence on the spectrum of these droplets—such as geometrical distortion and lasing, nondissolved tracer lumps, and similar phenomena. Although the focus is on the experimental technique itself, we supplement detailed studies of lasing in falling microdroplets. These results were obtained with the aim of developing a system for measuring temperature distributions in droplets and sprays. In the light of these results the practice of calibrating a droplets spectrum by use of a bulk liquid sample needs to be critically reviewed.

Figures (10)

Classical explanation of morphology-dependent resonances as a result of total reflection. Light beams originating from the inside of the dashed circle refract to the outside. Illustrating the discrete size parameter x are two paths for x=5 and x=15.

Spectral slicing: images and spectra. Positioning the entrance slit of the spectrograph x0 selects the x coordinate, whereas the software allows one to select only parts of the spatially resolved spectrum. The resulting spectrum originates only from within the cross section.

Four droplets resulting from four different aperture diameters of the droplet generator. To the left-hand side is the original calibration image taken with etched glass (linewidth 10μm). Below are the respective spectra, which were normalized at 410nm for comparison. The legend indicates the aperture diameter in micrometers. The ring of high intensity resulting from lasing is clearly visible in the images, as is the effect on the spectrum.

Effect of the absorption spectrum on the lasing spectrum of a single droplet of Rhodamin B in ethanol. The spectra may be compared only qualitatively, as the absorption spectrum was recorded with a bulk sample.

Lasing spectrum for POPOP droplets created by various aperture diameters. Fluorescence spectra are normalized at 410nm. For comparison emission and absorption spectrum of a bulk liquid sample are included, as is an estimate of the lasing threshold (unmodified; see text).

Lasing spectrum for α-NPO-droplets created by various aperture diameters. Fluorescence spectra are normalized at 430nm. For comparison emission and absorption spectrum of a bulk liquid sample are included, as is an estimate of the lasing threshold (modified with absorption spectrum).

Averaged droplet spectrum (aperture 50μm) and bulk liquid spectrum (α-NPO) together with standard deviation from the average. In the region dominated by lasing the signal fluctuates strongly, as indicated by a deviation of almost 50%.

Eliminating lasing in droplets by reducing the dye concentration (Rhodamin B). The graph shows a bulk liquid spectrum in comparison with droplet spectra from high and low concentration of Rhodamin B in ethanol.